Analyte determination method and analyte meter

ABSTRACT

The presence of oxygen or red blood cells in a sample applied to an electrochemical test strip that makes use of a reduced mediator is corrected for by an additive correction factor that is determined as a function of the temperature of the sample and a measurement that reflects the oxygen carrying capacity of the sample. The measured oxygen carrying capacity can also be used to determine hematocrit and to distinguish between blood samples and control solutions applied to a test strip.

BACKGROUND OF THE INVENTION

This application relates to a method for correcting for the presence ofoxygen in an electrochemical test strip that makes use of a reducedmediator, to a meter and meter-test strip combination that perform themethod in analyzing samples, and to a method and apparatus fordetermination of hematocrit. The invention also provides a method andapparatus for distinguishing between blood samples and control solutionsapplied to electrochemical test strips.

Small disposable electrochemical test strips are frequently used in themonitoring of blood glucose by diabetics. Such test strips can also beemployed in the detection of other physiological chemicals of interestand substances of abuse. In general, the test strip comprises at leasttwo electrodes and appropriate reagents for the test to be performed,and is manufactured as a single use, disposable element. The test stripis combined with a sample such as blood, saliva or urine before or afterinsertion in a reusable meter, which contains the mechanisms fordetecting and processing an electrochemical signal from the test stripinto an indication of the presence/absence or quantity of the analytedetermined by the test strip.

Electrochemical detection of glucose is conventionally achieved byapplying a potential to an electrochemical cell containing a sample tobe evaluated for the presence/amount of glucose, an enzyme that oxidizesglucose, such as glucose oxidase, and a redox mediator. As shown in FIG.1, the enzyme oxidizes glucose to form gluconolactone and a reduced formof the enzyme. Oxidized mediator reacts with the reduced enzyme toregenerate the active oxidase and produce a reduced mediator. Reducedmediator is oxidized at one of the electrodes, and then diffuses back toeither be reduced at the other electrode or by the reduced enzyme tocomplete the cycle, and to result in a measurable current. The measuredcurrent is related to the amount of glucose in the sample, and varioustechniques for determining glucose concentrations in such a system areknown. (See, U.S. Pat. Nos. 6,284,125; 5,942,102; 5,352,2,351; and5,243,516, which are incorporated herein by reference.)

One source of variability in the measurements achieved with such systemsis the amount of oxygen that is present in the sample. (See U.S. Pat.No. 6,251,260) Oxygen can interact with the reduced enzyme to regenerateoxidized enzyme. Since this process results in oxygen consumption andnot in the production of reduced mediator, glucose is consumed withoutgeneration of the charge carrier. As a result, the apparent glucosereading is low. Oxygen can also be consumed by reaction with reducedmediator to generate oxidized mediator, and this process also results inan artificially low value of glucose.

Because hemoglobin in red blood cells can act as a reservoir of oxygen,it has been proposed to structure electrochemical test strips so as toexclude red blood cells from the area where the enzyme and mediator arelocated. (See, for example, U.S. Pat. No. 6,241,862)

It has been suggested to make corrections to the glucose reading basedon a hematocrit determination on the sample. For example, U.S. Pat. No.6,475,372 discloses a method in which a sample is introduced into anelectrochemical cell having a working and reference electrode. A firstelectric potential is applied to the cell and the resultant cell currentover a first period of time is measured to determine a firsttime-current transient. A second electric potential of opposite polarityis then applied to the cell and a second time-current transient isdetermined. The preliminary concentration of the analyte (C0) is thencalculated from the first and/or second time-current transients. Thispreliminary analyte concentration, less a background value, is thenmultiplied by a hematocrit correction factor to obtain the analyteconcentration in the sample, where the hematocrit correction factor is afunction of the preliminary analyte concentration and the ratio of 2current values within the time-current transient of the electrochemicalcell. U.S. Pat. No. 6,287,451 discloses a method in which a hematocritcorrection is made to analyte concentration, and measure of hematocritis determined based on a measured resistance between a working electrodeand a reference electrode.

Correction for the presence of red blood cells can be based on either orboth of two approaches: a mobility-based approach that removes theaffects of hematocrit to produce a glucose measure that is independentof the physical aspects of red blood cells, and a chemical approach thataccounts for oxygen attrition which is related to the amount ofhemoglobin as an oxygen carrier. The first of these approaches isreflected in a difference in slope of the glucose calibration curve as afunction of hematocrit, and as such it is corrected by a multiplicativefactor. The second approach is reflected in an offset among glucosecalibration curves as a function of hematocrit and it is corrected withan additive factor. The present invention relates to corrections basedon the second approach, although it can be used in combination withcorrections based on the first approach.

SUMMARY OF THE INVENTION

The present invention relates to the electrochemical determination ofanalyte in systems using a reduced mediator as a charge carrier andprovides an additive correction for the presence of oxygen in thesample. In accordance with the invention, a method is provided fordetermining an analyte in a sample comprising the steps of:

(a) placing the sample in an electrochemical test cell comprising aworking and a counter electrode and a mediator that serves as a chargecarrier in the electrochemical determination of the analyte,

(b) electrochemically determining an uncorrected result for thedetermination of analyte in the sample;

(c) determining an additive correction factor for the amount of oxygenin the sample, and

(d) modifying the uncorrected result of step (b) with the correctionfactor of step (c) to provide a corrected determination of analyte inthe sample,

wherein the additive correction factor for the amount of oxygen in thesample is determined as a function of the temperature of the sample anda measurement that reflects the oxygen carrying capacity of the sample.

In an embodiment of the invention, a potential is applied to a sample inan electrochemical test cell, and an electrochemical signal is observedfrom which a determination of a raw analyte concentration can be made.The potential is then switched off, and the time required for thepotential between the electrodes to decay to a pre-defined level, forexample 50 mV, is determined. This time, tmob, is indicative of themobility of mediator in the sample. An additive correction factor isthen determined as a function of tmob and the temperature of the sample,and used to correct the raw analyte reading for oxygen in the sample.

In a further aspect of the invention, a meter for determination of ananalyte, such as glucose, that applies the additive correction forglucose is provided.

In a further aspect of the invention, a system comprising a meter fordetermination of an analyte, such as glucose, that applies the additivecorrection for glucose, and an electrochemical test strip is provided.

In a further aspect, the invention provides a method and apparatus fordetermination of hematocrit.

In a further aspect, the present invention provide a method andapparatus for distinguishing between a blood sample and a controlsolution applied to an electrochemical test strip.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to a drawing in severalfigures.

FIG. 1 shows the electron transfer reactions that occur in aconventional amperometric glucose detector.

FIG. 2 shows current versus time profiles observed in two differentelectrochemical test strip configurations, one with facing electrodesand one with side-by-side electrodes.

FIG. 3 shows a plot of voltage versus time, when applied voltage isswitched off.

FIGS. 4A-D show locations in the reaction pathway where oxygen attritioncan occur.

FIGS. 5A and 5B show the relationship between a measured indication ofanalyte concentration and tmob under various conditions of temperature,pO2 and glucose concentration.

FIG. 5C shows a plot of the slope of the lines in FIGS. 5A and B as afunction of temperature.

FIG. 6 shows an exterior view of a meter.

FIG. 7 shows connection of a test strip and connectors in a meter;

FIG. 8 shows a circuit diagram for switching between amperometric andpotentiometric modes.

FIG. 9 shows a circuit diagram for switching between amperometric andpotentiometric modes.

DETAILED DESCRIPTION OF THE INVENTION I. Definitions

As used in the specification and claims of this application, thefollowing definitions should be applied:

(a) “additive correction factor” refers to the numerical correction thatis added to a raw determination for analyte concentration to arrive at acorrected value. “Additive” encompasses the addition of a negativelysigned value, and thus is equivalent to addition or subtraction.Application of this additive correction factor results in an offset ofthe determined values.

(b) “analyte” refers to a material of interest that may be present in asample. In the present application, the examples use glucose as ananalyte, but the present invention is independent of both the type andamount of analyte. Accordingly, application to glucose detection systemsshould be viewed as merely a specific and non-limiting embodiment. Insome cases, there may be one or more intermediate species between theactual analyte and the mediator. Any such intermediate species are alsoreferred to herein as an analyte.

(c) “determination of an analyte” refers to qualitative,semi-quantitative and quantitative processes for evaluating a sample. Ina qualitative evaluation, a result indicates whether or not analyte wasdetected in the sample. In a semi-quantitative evaluation, the resultindicates whether or not analyte is present above some pre-definedthreshold. In a quantitative evaluation, the result is a numericalindication of the amount of analyte present. The term “determination ofan analyte” may encompass several steps, such that the measured speciesis related in amount to the analyte, even though it is not relatedchemically.

(d) “mediator” refers to a chemical species that is electrochemicallydetected. Numerous electron transfer mediators suitable for detection ofanalytes such as glucose are known, and include without limitation iron,ruthenium, and osmium compounds. In some embodiments of the invention,the mediator is produced through one or more reaction steps and isrelated to the concentration of the actual analyte, such as glucose. Thepresent invention is also applicable, however, to circumstances in whichthe detected chemical species is the reduced form of the analyte to bedetected, and this is also an embodiment of the invention.

(e) “mobility” refers to the mobility of the mediator in theelectrochemical test cell. Mediator mobility is a property of themediator itself, i.e., the diffusion coefficient, but is also dependenton other sample properties such as hematocrit and viscosity.

(f) “oxygen attrition” is the discrepancy between measured reducedmediator concentration and actual analyte concentration as a result ofthe action of oxygen, and is corrected using an additive correctionfactor.

(g) “oxygen carrying capacity” refers to the capacity of the sample tohold oxygen, in dissolved form and in a red blood cell reservoir.

(h) “predetermined” is used in this application to refer to amounts orvalues that are determined empirically for a particular meter or teststrip or meter/strip combination. The predetermined amounts or valueswill reflect an optimization for the needs of the user, taking intoaccount the confidence levels needed, and need not achieve the bestpossible results or 100% accuracy.

(i) “switching off” of the applied potential refers to the creation ofan open circuit that forces the current to be zero (by opening a switchor introducing a high impedance into the circuit) that allows a built-upchemical concentration gradient and ion adsorption in the double layerto determine the potential between the electrodes. This is not the samething as setting the voltage to zero volts.

(j) “t_(mob)” is a time determined experimentally during an analysisthat reflects the mobility of mediator in a particular sample in aparticular test cell. t_(mob) is the time after the applied potential isswitched off, that it takes for the potential between the electrodes todecay to a pre-determined value.

II. Oxygen as an Interferent in Electrochemical Reactions

The present invention is directed to solving the problem ofsample-dependent errors due to oxygen and to providing a correction forthese errors in the measurement of glucose and other oxidizable analytessuch as lactate, cholesterol and ethanol in samples where a reducedmediator is produced and serves as a charge carrier. The invention isapplicable to samples in which oxygen attrition is a concern,particularly where an oxygen reservoir, or other similar source ofresidual oxidation capability is present. In particular, the inventionis applicable to blood samples that contain red blood cells that act asan oxygen reservoir.

FIGS. 4A-D show a minimal representation of the reaction occurring in anelectrochemical cell, and the locations where oxygen attrition canoccur. FIG. 4A shows the basic reaction in which analyte is used togenerate mediator which is measured to produce a signal. The “chemicalchange” noted does not imply that the analyte and the mediator arestructurally related, but indicates only a chemical process, such as oneor more electron transfer reactions, such that mediator is generated asa result of the presence of analyte.

FIG. 4B indicates that oxygen attrition can occur as a result of oxygeninteraction with the chemical change, for example oxidation of enzyme.FIG. 4C indicates that oxygen attrition can occur as a result of oxygeninteraction with the mediator. Finally, as shown in FIG. 4D, bothprocesses may occur where oxygen availability is high or the rates ofthe two processes are similar. Any of the processes depicted in FIGS.4-D introduce error into the assumption that [analyte]∝[mediator], andresults in a reduction in the amount of observed current and a lowreading for glucose.

Oxygen is present in blood samples as dissolved oxygen in equilibriumwith oxygen bound to hemoglobin. Thus, the hemoglobin in red blood cellsacts as reservoir for oxygen, and oxygen-related error may be greater insamples with high hematocrit.

Direct measurement of available oxygen in a blood sample within a lowcost disposable test strip is not feasible. Furthermore, the reaction ofO₂ with reduced mediator does not necessarily proceed to completion,i.e. to consumption of all O₂. Thus, merely knowing the amount of O₂ maynot yield an appropriate correction factor. The reaction is related tothe availability of O₂ however, and certain aspects of the oxygenavailability are accessible for measurement, and these are utilized inthe present invention. Oxygen availability can be represented generallyby the equation:O₂ availability=f(O₂ activity, amount of O₂)

and may be given byO₂ availability=O₂ activity×amount of oxygen

in an idealized case, where O₂ activity is a function of temperature andpO₂ and the amount of O₂ is a function of the oxygen carrying capacity(OCC) of the sample.

III. Electrochemical Test Cells Useful in the Invention

The present invention is applicable to electrochemical test cells,including disposable test strips with the following characteristics:

(1) at least two electrodes, a working and counter electrodes; and

(2) an enzyme that oxidizes the analyte and a redox mediator thatoxidizes the reduced enzyme.

In addition to the characteristics listed above, the electrochemicaltest strip can include means for determining the temperature of thesample. This can be, for example a liquid crystal strip, such onesupplied by Temperature Technology, Inc. of Adelaide, South Australia,which is described at www.t-tec.com.au/2003/thermistors/LiquidC.html/.Alternatively, the means for determining the temperature of the samplecan be part of the meter, such that it is reused, rather than part of adisposable strip. When the temperature sensing device is part of themeter, large and more expensive components can be reasonably employed,for example thermistors, and thermocouples.

Further, electrochemical test strips useful in the invention can includeother additional features that are not necessary to the determination ofoxygen carrying capacity as described herein, but which may be useful indetermining analyte. For example, the electrochemical test strip mayinclude one or more additional electrodes which can be referenceelectrodes, electrodes for determining sufficiency of sample volume, orelectrodes for determining the time of sample application.

IV. Determination of Analyte, Such as Glucose

There are a variety of known methods for electrochemically determininganalytes such as glucose in samples, and the present invention whichincludes an additive correction for oxygen attrition can be employed inthe context of any of these methods. The following discussion isprovided by way of non-limiting example.

FIG. 2 shows current versus time profiles observed in two differentelectrochemical test strip configurations, one with facing electrodesand one with side-by-side electrodes, where the electrochemical reagentsare initially disposed only on the working electrode, and not on thecounter electrode. In both cases, the current trace shows an immediateinitial current 21 on the time scale shown following application of thepotential. This current is associated with the initial charging of thedouble layer at the surface of the electrodes. Thereafter, the currentdecreases, because current is dependent on the mediator diffusing fromthe working electrode to the counter electrode. The duration of thisreduced current (indicated by arrow 20) is dependent on the distancebetween the electrodes, and on the mobility of the mediator. After theperiod of reduced current 20, the current rapidly rises to a peakcurrent 22. In the case of facing electrodes, the current declines to aplateau current 23 which reflects the recycling or shuttling of mediatorbetween the electrodes. In the case of side-by-side electrodes, thecurrent continues to decay in the time scale indicated, as indicated bydashed line 24. At longer times, this curve 24 also shows effects ofrecycling/shuttling of mediator.

In the region of the decay following the peak, before recycling becomesdominant, the current decay can be modeled by the Cottrell equation,i.e.,1/I ² ∝t

where I is the current and t is time. The square root of the slope of1/I² versus t is a parameter called the Cottrell slope. Cottrellanalysis can be utilized to determine glucose concentration as describedin U.S. Pat. Nos. 5,243,516; 5,352,351 and 6,284,125.

The present invention can also be used in combination with otherelectrochemical techniques that determine analyte concentration. Forexample, the analyte concentration can be detected using coulometricanalysis, as described in U.S. Pat. No. 6,592,745, which is incorporatedherein by reference; or using a conduction cell plateau current, asdescribed in U.S. patent application Ser. No. 10/924,510, which isincorporated herein by reference.

V. Determination of Additive Correction Factor

The additive correction factor can be assessed by any technique thatgives a measure of oxygen carrying capacity, in combination with atemperature measurement for the sample. The present inventors have foundthat a graph of measured raw analyte concentration versus a measure ofoxygen carrying capacity is a line with a slope that is dependent on thetemperature at which the measurements are made, but that is independentof pO₂ and glucose concentration over normal ranges of values. Changesin pO₂ or glucose concentration result in an additive offset of thegraphed lines, but not a change in slope. (See FIGS. 5A and B) A plot ofthis slope as a function of temperature (See FIG. 5C) can be used todefine slope (S) and intercept (I) parameters that are combined into theadditive correction factor of the invention for a given temperature T,in accordance with the equation:additive correction factor=constant×[(S×T)+I]×OCC

where OCC is a measure of oxygen carrying capacity such as hematocrit,and the constant is an empirically determined factor with a positive ornegative sign.

Accuracy of the additive correction factor can be improved when there isa large body of data gathered at one temperature and a limited body ofdata gathered at the measurement temperature by determining only theslope from the data gathered at the measurement temperature anddetermining the intercept from all of the available data. Thus, in thecase where a large body of standard calibration data is available forthe parameter I may be a constant established for the strip and metercombination, and only the slope need to be determined experimentally.

(a) Use of t_(mob) as a measure of oxygen carrying capacity

In one embodiment of the invention, t_(mob), a measure of the mobilityof the mediator is used as the measure of oxygen carrying capacity.

After measurements sufficient to allow determination of a raw analyteconcentration, the applied potential is switched off at a timet_(switch). At this point, a chemical potential gradient exists betweenthe electrodes as a result of the unequal distribution of oxidized andreduced mediator. This chemical potential gradient decays over timeafter the potential is switched off, and the rate of this decay isdependent on the mobility of the mediator in the sample. (See commonlyassigned U.S. patent application Ser. No. 10/924,510, which isincorporated herein by reference.) FIG. 3 shows the applied potential,V_(app), and the measured potential at the electrodes, V_(elect), as afunction of time (in arbitrary units), commencing at a time after thecurrent measurement that is used in the apparatus to determine rawanalyte concentration. Note that the conversion of measured current to ameasurement of raw analyte concentration need not be done prior to theswitching off of the potential.

In accordance with one embodiment of the present invention, the decay inpotential is monitored until the observed potential has decreased to apre-determined value, V_(mob). Decreases to around 50 mV are convenientwhere the applied voltage is on the order of 300 mV, although somewhatsmaller values such as 47 mV or 48 mV may be found to provide optimumresults in particular experimental configurations. In general, V_(mob)is suitably 0.025 to 0.1V For example, in glucose determinations with aV_(app) of 250 to 300 mV, V_(mob) is suitably in the range of 25 to 100mV, preferably 45 to 50 mV.

The time at which this drop has occurred is noted on FIG. 3 as t_(meas)and t_(mob) is given by:t _(mob) =t _(meas) −t _(switch).

Other ways of determining a measure of the rate of decay may also beemployed. For example, an instantaneous slope of the decay of thepotential can be determined, or the decrease in voltage over apredetermined time can be used. The meter may also select a particulartime window and perform a linear regression on V versus log(t) or ln(t)to find t_(mob) which is the time to a particular voltage. If theV_(mob) does not fall within the selected window, a projection based onthis linear fit can be used. The specific methodology is not critical,provided that the value of the measured decay is taken into account indetermining the correction function.

(b) Use of Other Techniques as a Measure of Oxygen Carrying Capacity

U.S. Pat. Nos. 6,287,451 and 6,475,372 discussed above discloseelectrochemical methods for determination of hematocrit in a disposabletest strip. The hematocrit measurement is used in a multiplicativecorrection, as opposed to the additive correction of the presentinvention. The measurement can be used in both modes, however, just ast_(mob) is used for both types of corrections as described above. Thisis because hematocrit is a measure of the red blood cells, and red bloodcells have an oxygen carrying capacity.

In order to use any type of hematocrit measurement in present invention,a series of calibration measurements are taken to obtain data pointpairs of uncorrected analyte concentration and hematocrit at each of aplurality of temperatures. At each temperature, the data points are fitto a linear model and the slope of the line is determined. As notedabove, this slope is independent of glucose and pO₂ such that whilethese parameters need to be kept the same across experiments, theparticular values are not significant. The resulting slope/temperaturedata point pairs are then fitted to a linear model, to determine theslope and intercept which is incorporated into an additive correctionfactor as described above.

In some cases, the linear model may be sufficient only for a narrowrange of the data. An improved additive correction factor may bedetermined for a wider range of temperatures or oxygen carryingcapacities by introducing non-linear terms such as quadratic equationsof exponents to terms.

VI. Correction of Analyte Value

In the method of the invention, correction for the chemical effects ofoxygen is done by adding an offset to the uncorrected analyte reading,to produce a corrected value. Additional corrections can be employed incombination with the additive correction of the invention. For example,multiplicative corrections relating to physical hematocrit effects,calibration corrections specific for individual lots of test strips, andcorrections for other interferents may also be made. In addition, as iswell known in the art, look-up tables or other conversion mechanisms canbe used to convert determined signals into user-understandable values ofanalyte concentration in defined units.

VII. Dynamic Switching from Amperometric to Potentiometric Mode

In the present application, the meter first acts in an amperometricmode, and then after the applied potential is switched off, in apotentiometric mode. In order to enhance the quality and consistency ofmeasurements made when operating in potentiometric mode, if is desirableto perform the switch to potentiometric mode only after a stablediffusion gradient of oxidized and reduced mediator has formed withinthe electrochemical test cell. In general, the potentiometrymeasurements will give the same stable reading at any point after theconcentration gradients have formed a stable profile that extends “farenough” into the bulk of the sample.

To maximize the chances that stable diffusion gradients have beenachieved, it is possible to simply establish a time after the start timeof the measurement cycle at which the switch will be made. This time isdetermined empirically for a given test strip design, but may generallybe on the order of 4 to 8 seconds. To allow the meter to accommodate avariety of different sample characteristics, however, t_(switch) can bedetermined dynamically.

In one embodiment of the invention, t_(switch) is determined dynamicallyfrom the determined value of t_(peak) (the time of peak 22, in FIG. 2)by adding a time interval, for example 2 to 3 seconds to the determinedvalue of t_(peak).

In another embodiment of the invention, t_(switch) is determineddynamically using a fixed value of t_(switch) when t_(peak) is small andt_(peak) plus a predetermined amount when t_(peak) is larger. Forexample t_(switch) may have a fixed value of 3.5 second when t_(peak) isless than 1.5 seconds, and be equal to t_(peak) plus an offset (forexample 2 second) when t_(peak) is greater than 1.5 seconds.

In yet another embodiment, a third mode for measurement is establishedfor circumstances when t_(peak) occurs at times that are longer thanordinary. In this case, when t_(peak) occurs above a predeterminedthreshold, for example 5 seconds, t_(switch) is suitably determined as afunction of t_(peak) and an additive correction factor that usespredetermined constants derived from the slope of the Cottrell current.

Further, a maximum value of t_(peak) can be established above which anerror message is generated.

VIII. Apparatus of the Invention

The method of the invention can be used with any strip as describedabove, provided that a meter apparatus is providing that can receive thestrip and provide the necessary applications of voltage and signalprocessing. Such a meter also forms an aspect of the present invention.Thus, the invention provides a meter for receiving an electrochemicaltest strip having electrodes and providing a determination of an analytein a sample applied to the electrochemical test strip when received inthe meter, said meter comprising

(a) a housing having a slot for receiving an electrochemical test strip;

(b) communications means for receiving input from and communicating aresult to a user; and

(c) means for determining a raw analyte concentration value, and fordetermining an additive correction factor for the amount of oxygen inthe sample, and modifying the raw analyte concentration to provide acorrected determination of analyte in the sample, wherein the additivecorrection factor for the amount of oxygen in the sample is determinedas a function of the temperature of the sample and a measurement thatreflects the oxygen carrying capacity of the sample.

When not present in the strip, the meter of the invention also includesa means for measuring the temperature of the sample in the meter, forexample a thermistor or thermocouple. FIG. 6 shows an external view of ameter in accordance with the invention. The meter has a housing 61, anda display 62. The housing 61 has a slot 63, into which a test strip isinserted for use. The meter may also have a button 64 for signaling thestart of the measurement cycle, or may have an internal mechanism fordetecting the insertion of a test strip or the application of a sample.Such mechanisms are known in the art, for example from U.S. Pat. Nos.5,266,179; 5,320,732; 5,438,271 and 6,616,819, which are incorporatedherein by reference. In the meter of the invention, buttons, displayssuch as LCD displays, RF, infrared or other wireless transmitters, wireconnectors such as USB, parallel or serial connections constitute meansfor receiving input from and communicating a result to a user, and canbe used individually and in various combinations.

FIG. 7 shows an interior view in which the connection of the meter to atest strip is shown. As shown, the test strip 71 has contacts 72, 73 bywhich the electrodes are placed in electrical contact with contacts 74,75 of the meter.

The means for determining a raw analyte concentration value, and fordetermining an additive correction factor for the amount of oxygen inthe sample, and modifying the raw analyte concentration to provide acorrected determination of analyte in the sample comprises circuits,such as on a circuit board, associated with a programmed microprocessorthat interacts with the circuits to provide the desired switchingbetween amperometric and potentiometric modes and to monitor current andvoltage as described. Apparatus suitable for switching between anamperometric mode of operation in which current is measured and apotentiometric mode of operation in which a potential difference betweenthe electrodes is measured are described in commonly assigned U.S.Provisional Applications Nos. 60/521,592, filed May 30, 2004, and60/594,285 filed Mar. 25, 2005, and commonly assigned U.S. patentapplication Ser. No. 10/907,790, filed Apr. 15, 2005, which areincorporated herein by reference.

FIG. 8 shows an electrical schematic of the amperometric/potentiometricportion of one embodiment of the meter of the invention. It will beappreciated, however, that other components can also be used, whichachieve the same results in terms of applying and switching the voltage.Working electrode 80 is connected to op amp 81 via a connectorcontaining switch 82, and to op amp 83. Counter electrode 84 isconnected to op amps 85 and 86. Op amps 83, 85 and 86 are high impedanceinput amplifiers. When operating in amperometric mode to determine ananalyte, a voltage V₂ is applied to op amp 81, and a voltage V₁ isapplied to op amp 85, V₂ being greater than V₁. The resulting potentialdifference between the electrodes results in the generation of a currentthat is related to the amount of analyte, and this current can bemonitored at output 87 and converted to an indication of the presence oramount of analyte. When switch 82 is opened to create an open circuitand stop application of the potential difference, current flow ceases,and the output of amplifier 86 assumes the potential of the counterelectrode, while the output of amplifier 83 assumes the potential of theworking electrode 80. The difference between the output from op amp 83and op amp 86 indicates the decay in chemical potential and is processedin accordance with the methods described above to determine the mobilityof the mediator.

FIG. 9 shows an alternative version of this circuit using only two opamps and an increased number of switches. Working electrode 80 isconnected to op amp 81 which receives input voltage V₂. Counterelectrode 84 is connected to high input impedance op amp 90 via one oftwo switched paths. Input voltage V₁ is connected to the circuit via athird switched path. When switch 91 and 93 are closed, and switch 92 isopen, the circuit functions in amperometric mode, and the output at 95reflects current flow at the electrodes. When switch 92 is closed, andswitches 91 and 93 are open, the circuit operates in potentiometric modeand the output at 95 assumes the potential of the counter electrode(similar to amplifier 86 in FIG. 8). Thus, the output at 95 indirectlyreflects the difference in potential between the electrodes. The actualdifference in potential between the electrodes is the difference betweenthe output at 95, and the output of op amp 81 (at 80, the workingelectrode).

In one embodiment of the invention, the apparatus fits stored datapoints (t,V) extending from t_(switch) to a time after the expected timet_(meas) to the model equation V(t)=a₁×ln(t)+a₂ by a least squaresregression. The resulting values of a₁ and a₂ are then used to calculatethe value of t_(meas) at which the potential has fallen to V_(mob)according to the equation

$t_{meas} = {\mathbb{e}}^{(\frac{V_{mob} - a_{1}}{a_{2}})}$

The apparatus retrieves stored values for the slope and intercept of theraw concentration/temperature plot and the constant, determined fromcalibration runs using the same meter and test strip configuration andcombines these values with the determined t_(mob) to calculate thecorrection factor according to the equation:additive correction factor=constant×[(S×T)+I]×t _(mob)

This additive correction factor is then added to the raw analyteconcentration.

IX. Measurement System

In actual use, the meter described above is combined with anelectrochemical test strip for the determination of a particularanalyte, such as glucose. This combination, referred to as a measurementsystem, forms a further aspect of the present invention.

X. Determination of Hematocrit

While the invention has been defined to this point in terms of making acorrection in the measurement of an analyte, the disclosed determinationof t_(mob) can also be used independently, with or without acontemporaneous determination of an analyte, to assess the hematocrit ofa blood sample. Thus, in accordance with a further aspect of the presentinvention a method is provided for determining hematocrit comprising thesteps of:

(a) introducing a blood sample between first and second electrodes in anelectrochemical test cell containing a redox active species that servesas a charge carrier;

(b) applying a potential for period for time sufficient to establish achemical potential gradient between the electrodes,

(c) switching off the applied potential at a time t_(switch), andobtaining a value indicative of the rate of decay of the chemicalpotential gradient in the absence of applied potential, and

(d) comparing the determined value of the rate of decay with a standardcurve relating rate of decay to hematocrit at the determined temperatureto arrive at a value for the hematocrit of the sample. One suitablemeasure of the rate of decay is t_(mob) as described above. A morerefined measure of hematocrit may be obtained by also determining thetemperature and using a value like the additive correction factordescribed above for comparison.

The redox active species in this aspect of the invention can be amediator, which interacts with glucose and glucose oxidase as describedabove, or with some other redox active species that is inherentlypresent in blood. Particularly where no determination of analyte isbeing made, however, the redox-active species may also be an addedmaterial, such as a mixture of ferricyanide and ferrocyanide ions, whichcan give current at the positive electrode by oxidation of ferrocyanideand at the negative electrode by reduction of ferricyanide at theapplied potential.

The apparatus for measuring hematocrit differs from the meter in threesignificant respects. First, the programming for determining an analyteand displaying the results are optional. Second, the programmingincludes a look up table or other conversion mechanism for taking themeasured value of the rate of decay and the optionally the temperatureand converting it into a value for hematocrit in conventional units.Third, the programming provides for display of the hematocrit value.

XI. Distinguishing Samples from Control Solutions

Some known analyte test meters have included the ability toautomatically distinguish between strips to which a blood sample isapplied and strips to which a control solution (for example analytesolution in water, viscosity-adjusted analyte solutions, or plasmacontaining a known amount of analyte) is applied. (See for example U.S.Pat. Nos. 6,645,368 and 6,824,670). The measurement of t_(mob) asdescribed herein, with or without consideration of temperature can alsobe used for this purpose.

Thus, in accordance with a further embodiment of the invention, a methodand apparatus are provided in which material is applied to the teststrip disposed in the apparatus, the material is identified as a bloodsample or a control solution, and the signal from the test strip isprocessed as test sample or a calibration run based on thisidentification, wherein, the identification of the material as a sampleor control solution is performed by the steps of:

(a) applying a potential for period for time sufficient to establish achemical potential gradient between the electrodes,

(b) switching off the applied potential at a time t_(switch), andobtaining a value indicative of the rate of decay of the chemicalpotential gradient in the absence of applied potential, and

(c) comparing the determined value of the rate of decay with a thresholdvalue, wherein a value on one side of the threshold value indicates thatthe material is a blood sample, and a value on the other side of thethreshold value indicates that the material is a control solution.

In one embodiment of this aspect invention, the square root of thedetermined t_(mob) is compared to a threshold value. The specificnumerical threshold value will depend on the meter and the strip, andcan be determined by testing a plurality of blood samples and controlsolutions and determining the threshold value that distinguishes betweenthe two data sets with the desired degree of confidence. For example, alonger time measurement t_(mob) or a slower rate than the thresholdvalue is indicative a blood sample.

This measurement of hematocrit can also be used to provide patient/userwith an indication of anemia or other abnormal levels of red bloodcells. In such an embodiment, a medically appropriate definition ofanemia is used to define a threshold. The processor is set to comparethe determined hematocrit value with this threshold, and provide anindication of anemia or normal red blood cell count based on the resultof the comparison. If desired, intermediate thresholds can be used toestablish degrees of red blood cell deficiency for bettercharacterization of borderline cases. Since the same measurements arebeing used that are used for analyte determination, measurements ofhematocrit, or anemia warning indications can be provided in a meterdedicated for this purpose, or in a combination meter that also testsfor an analyte such as glucose.

XII. Examples

The invention will now be further described with reference to thefollowing non-limiting examples. In these examples, measurements weremade using electrochemical test strips having facing screen printedcarbon electrodes, a nominal sample volume of 625 nanoliters, and aviewing window. Blood samples used in the tests were freshly drawn (lessthan 8 hours old) using Vacutainer™ tubes, and were stabilized with EDTAas an anticoagulant. Blood samples with various hematocrits wereprepared by centrifuging a blood sample of known hematocrit of 40 andknown glucose concentration, removing enough plasma to leave a hct 60sample, and then creating a lower hematocrit of 20 by mixing equalvolumes plasma and hct 40 blood. Because these samples were all preparedrapidly from a single blood sample, they all have the same plasmaglucose concentration. Different glucose concentrations were generatedby adding amounts of 1 M glucose stock solution to blood prior tocentrifugation.

After application of the blood sample, 300 mV was applied to the stripsfollowing a profile as shown in FIG. 3. t_(mob) was determined based ona V_(mob) of 47 mV. The slope in the Cottrell region of the currentversus time plot (see FIG. 2) was determined and an “uncorrected” valuefor glucose concentration (as inferred from the actually measuredmediator concentration) was determined in the meter as

$\sqrt{\frac{t_{mob}}{Cottrellslope}}$

This value already includes a mobility-type physical correction.

In referring to the measured quantity as an uncorrected or raw glucoseconcentration, it should be understood that what was actually measuredin this instance was reduced mediator (the charge carrier at theelectrodes), and that this measurement is converted (using a look uptable and/or calibration values in the meter) to a glucose value.

FIG. 5A shows results obtained at a single glucose level and twodifferent values for pO₂ when samples were run at three differenttemperatures. While each of the lines in the graph is different, theimportant observation for purposes of this invention is that at anygiven temperature, the lines for different pO₂ values have the sameslope. They are simply offset. For example, line 501 which is the coldtemperature, low pO₂ line has the same slope as line 502, the coldtemperature, medium pO₂ line.

FIG. 5B shows similar results obtained at a single pO₂ level and twodifferent values for glucose (˜3.5 mM and ˜9 mM) when samples were runat three different temperatures. Again, while each of the lines in thegraph is different, the important observation for purposes of thisinvention is that at any given temperature, the lines for differentglucose values have the same slope. They are simply offset. For example,line 510 which is the cold temperature, high glucose line has the sameslope as line 511, the cold temperature, low glucose line.

FIG. 5C shows a graph of the slope of each of these lines as a functionof temperature, fit to a linear model. This plot was used to determinethe slope and intercept values (S and I) for the meter and test stripcombination employed. These values are used to generate the additivecorrection factor according to the formula:additive correction factor=constant×[(S×T)+I]×t _(mob)

In the meter and strips employed for this testing, using the uncorrectedthe analyte concentration as described above, the constant is −1.

Those skilled in the art will have no difficulty devising myriad obviousimprovements and variations of the invention, none of which depart fromthe invention and all of which are intended to be encompassed within thescope of the claims which follow.

1. A method for determining hematocrit of a blood sample comprising thesteps of: (a) introducing a blood sample between first and secondelectrodes in an electrochemical test cell containing a redox activespecies that serves as a charge carrier; (b) applying a potential forperiod for time sufficient to establish a chemical potential gradientbetween the electrodes, (c) switching off the applied potential at atime t_(switch), and obtaining a value indicative of the rate of decayof the chemical potential gradient in the absence of applied potential,and (d) comparing the determined value of the rate of decay with astandard curve relating rate of decay to hematocrit to arrive at a valuefor the hematocrit of the sample.
 2. The method of claim 1, wherein thevalue indicative of the rate of decay of the chemical potential gradientis a time t_(mob), which is determined by determining the time,t_(meas), required for the potential to decay to a pre-determinedvoltage t_(mob), and wherein t_(mob)=t_(meas)−t_(switch).
 3. Anapparatus for measurement of hematocrit comprising: (a) a housing havinga slot for receiving an electrochemical test strip; (b) communicationsmeans for receiving input from and communicating a result to a user; and(c) means for applying a potential for period for time sufficient toestablish a chemical potential gradient in a blood sample receivedbetween the electrodes, switching off the applied potential at a timet_(switch), and obtaining a value indicative of the rate of decay of thechemical potential gradient in the absence of applied potential, and (d)means for comparing the determined value of the rate of decay with astandard curve relating rate of decay to hematocrit to arrive at a valuefor the hematocrit of the sample.
 4. The apparatus of claim 3, whereinthe value indicative of the rate of decay of the chemical potentialgradient is a time t_(mob), which is determined by determining the time,t_(meas), required for the potential to decay to a pre-determinedvoltage V_(mob), and wherein t_(mob)=t_(meas)−t_(switch).
 5. Theapparatus of claim 4, wherein the apparatus fits stored data points(t,V) extending from t_(switch) to a time after the expected timet_(meas) to the model equation V(t)=a₁ X ln(t)+a₂ by a least squaresregression, and wherein the resulting values of a₁ and a₂ are used tocalculate the value of t_(meas) at which the potential has fallen toV_(mob) according to the equation$t_{meas} = {{\mathbb{e}}^{(\frac{V_{mob} - a_{1}}{a_{2}})}.}$
 6. Amethod for determining an indication of anemia using a blood samplecomprising the steps of: (a) introducing a blood sample between firstand second electrodes in an electrochemical test cell containing a redoxactive species that serves as a charge carrier; (b) applying a potentialfor period for time sufficient to establish a chemical potentialgradient between the electrodes, (c) switching off the applied potentialat a time t_(switch), and obtaining a value indicative of the rate ofdecay of the chemical potential gradient in the absence of appliedpotential, and (d) comparing the determined value of the rate of decaywith a standard value for the rate of decay corresponding to a thresholdvalue indicative of anemia.
 7. The method of claim 6, wherein the valueindicative of the rate of decay of the chemical potential gradient is atime t_(mob), which is determined by determining the time, t_(meas),required for the potential to decay to a pre-determined voltage V_(mob),and wherein t_(mob)=t_(meas)−t_(switch).
 8. An apparatus for assessmentof anemia comprising: (a) a housing having a slot for receiving anelectrochemical test strip; (b) communications means for receiving inputfrom and communicating an assessment of anemia to a user; and (c) meansfor applying a potential for period for time sufficient to establish achemical potential gradient in a blood sample received between theelectrodes, switching off the applied potential at a time t_(switch),and obtaining a value indicative of the rate of decay of the chemicalpotential gradient in the absence of applied potential, and (d) meansfor comparing the determined value of the rate of decay with a standardvalue a standard value for the rate of decay corresponding to athreshold value indicative of anemia.
 9. The apparatus of claim 8,wherein the value indicative of the rate of decay of the chemicalpotential gradient is a time t_(mob), which is determined by determiningthe time, t_(meas), required for the potential to decay to apre-determined voltage V_(mob), and wherein t_(mob)=t_(meas)−t_(switch).10. The apparatus of claim 3, wherein the predetermined amount is from25 to 100 mV.
 11. The apparatus of claim 3, wherein the predeterminedamount is from 45 to 50 mV.
 12. The apparatus of claim 3, furthercomprising means for measuring the temperature in the test cell.
 13. Themethod of claim 1, wherein the predetermined amount is from 25 to 100mV.
 14. The method of claim 1, wherein the predetermined amount is from45 to 50 mV.
 15. The method of claim 1, further comprising the step ofmeasuring the temperature in the test cell, wherein the standard curverelating rate of decay to hematocrit that is used to arrive at a valuefor the hematocrit of the sample is selected based on the measuredtemperature.